The present invention relates generally to adaptive noise suppression and, more particularly, to adaptive noise cancellation processing and comparative analysis for operating equipments or machinery. The invention relates especially to such apparatus which performs such adaptive cancellation processing and comparative analysis continuously in real time.
Although conventional performance measurements may provide an indication of defects or potential defects in operating equipments of machinery, typically only gross or catastrophic defects will be identified. Relatively few latent defects are discovered by such measurements. However, the acoustic signals emanating from an operating apparatus (acoustic signature) may contain information which identifies not only the present defects which could be found by performance measurements but also latent defects or potential defects which could lead to subsequent failures of the apparatus. Acoustic-frequency signals radiated from operating apparatus, for example apparatus having moving parts, can contain valuable information with respect to the present and future performance of the apparatus.
However, the acoustical analysis of operating components can be a difficult task. Background noise at acoustic-frequencies can mask the acoustical signals radiated from the component being examined. In the testing of a component in a factory environment for example, noise from test-stand machinery, noise from other components under test, and ambient factory noise may obscure the signals of interest radiating from the component under test. It may be possible to isolate the desired acoustical signatures using sound intensity techniques; however, in an environment where speed of analysis is important, an unacceptable amount of time is required to perform an acoustic intensity analysis due to the large number of measurements necessary to achieve sufficient accuracy. In cases where the background noise varies greatly or is unpredictable, sound intensity techniques will generally be unsatisfactory. Isolating and analyzing the acoustical signature of operating equipment is, in general, a difficult problem in any situation where an uncontrolled operating environment exists or where large amounts of interfering signals are present.
U.S. Pat. No. 4,658,426 entitled "Adaptive Noise Suppression" issued to Chabries et al. suggests that adaptive processing can be used to diminish the adverse effects of noise distortion in processing signals from machinery wherein the desired signal provides an indicia of wear or malfunction. The basic theory of adaptive noise cancellation is well known and has been described by Bernard Widrow et al. in the article "Adaptive Noise Cancelling: Principle and Applications", Proceedings of the IEEE, Vol. 63, No. 12, December 1975, pages 1692-1716. In particular, Widrow et al. describe a generalized multiple-reference noise canceler on pages 1710-1711. In adaptive noise canceling, a signal channel and at least one reference channel are needed to provide noise cancellation. The signal channel receives a signal of interest which is corrupted by additive noise or interference. The reference channels receive noise that is correlated in some way with the additive noise or interference on the signal channel but not substantially correlated with the signal of interest. The signals which are received by the reference channels are adaptively filtered and then subtracted from the signal received by the signal channel to obtain an estimate of the signal of interest.
However, the successful application of adaptive noise cancellation to the analysis of a dynamically changing acoustic signature of apparatus operating in an environment contaminated with dynamically changing background noise has proven difficult to achieve. The adaptive processor must be able to respond to background noise which may vary widely in amplitude, frequency, and duration. Similarly, the acoustic signal of interest may vary widely in amplitude, frequency and duration. The noise signals may be very similar to the signal of interest. These obstacles facing the cancellation apparatus are especially troublesome if the processing and analysis is required to be done in real time.
Consider the case where it is desired to perform acoustic signature analysis on automotive transmissions as part of end-of-the-line quality control testing in a factory environment. The background noise may be the entire range of factory noises including any acoustic interference generated by the test fixture itself for the transmission under test and identical test fixtures for performing simultaneous testing on other transmissions. This may include the acoustic signals generated by the other transmissions undergoing test or other acoustic signals generated by the transmission under test itself. If the test is to be performed on every transmission, the entire test procedure, including acceptance or rejection of the part, must be accomplished very quickly in order to maintain the speed of the assembly line, for example in 20-30 seconds. Acoustic analysis of the transmission may be required at several different rotational speeds or revolutions-per-minute (RPM's). In one test configuration, the transmission is driven continuously from rest to the maximum test RPM and back to rest during the allotted time. The adaptive canceler apparatus must be able to provide the acoustic signal of interest for selected RPM's and must be able to capture this signal as the operating speed of the transmission passes through the selected RPM's. In terms of the frequency domain in the case of an automotive transmission, the acoustic signals of interest may vary from 10 Hz to 5 kHz and may be have a bandwidth as narrow as a few Hertz or as wide as thousands of Hertz. In the time domain, the frequencies of interest may vary from 100 milliseconds to 200 microseconds in duration. The interfering noise signals may present an even larger variation in signals to be processed. The analysis of other devices may present even more stringent requirements.
To satisfy the foregoing requirements, the adaptive canceler must be able to perform continuous real-time cancellation of widely varying background noise at very high data rates over a broad spectrum of frequencies and signal durations. The background noise may be such that multiple reference channels are required for satisfactory cancellation. In a typical situation, after the background noise has been canceled and the canceler is providing a continuous stream of real-time data corresponding to the signal of interest, it is desirable to provide real-time analysis of the data. Automatic analysis of the data is preferred, or even required as is the case in a production line test of a device such as an automatic transmission, or in situations where it is important that the analysis be accomplished quickly or almost instantaneously. The adaptive canceler and the automatic analysis system must have a throughput rate capable of handling the great volume of data produced in these circumstances.